The digitizing of the scans of objects and pictures produces signals obtained by means of data capture equipment, that is known as a scanner. Digitizing is necessary if a picture is to be stored in a computer and it is desired to process the image. The two-dimensional analog information obtained in a run is analyzed into picture points or pixels, which have a differential grayscale. The application of digitizing to image processing is extensive and has come to embrace CAD, CAE, mapping, advertising, art, simulation, and almost all spheres of chemistry and physics. The requirements thereby placed on equipment are correspondingly high. In seeking an image capture of highest quality, information losses come directly and inevitably to the fore. These losses arise through the necessary splitting up of two-dimensional analog information into individual pixels having a known size and predefined distance from one another, wherein the pixels vary in intensity over an established grayscale. In later image processing in the computer there is undertaken an effort to confine the information losses within tolerable limits by means of mathematical algorithms.
Although today's applied mathematical algorithms bring astonishing results, they cannot regain in the image signal more information than was at hand in the object as scanned. It must therefore be the goal to limit, through an optimal data capture, the need for later image processing. This problem is not new, and it has been worked on for a long time.
From the German Patent 36 30 739 there is known a method in which, during data capture of an image, a shifting of the exposed detector array is effected. This shifting is very small and permits image capture in the unrecorded spaces between the individual detector elements of the detector array. The doubled data cluster that has been thereby obtained in scanning the object permits a better and more precise reconstruction of the analog picture information in the computer. The doubled data cluster, however, certainly increases the processing time for both the data capture and the processing in the computer.
In the same direction are aimed attempts at increasing the number of picture points (pixels) used for data capture. Apart from the technological difficulties in the production of a detector array having many detector elements, there arises a further difficulty, at the least a diminution of the area of a single detector which would normally produce a stronger output, or in any case a longer one.
The scanning of bigger images is achieved in practice through scanning of a number of smaller image portions. These portions must be reintegrated and processed in such a way that the raster scan of the first image is joined to the raster scan of the second image, etc. In order to achieve a uniform raster scan, there must be produced a uniform raster in accordance with image processing methods appropriate to the class of engineering involved by means of mathematical algorithms, at great cost of time. The raster produced in this manner has a deviation in comparison with a real raster of a picture, since interpolation and weighting require computation in the computer. All known methods of digitizing of pictures have not satisfied up to now this geometrical problem in the case of data capture.
A similar problem exists in the case of the analysis in photogrammetry of aerial photos, as it is not always possible to place the picture to be scanned in exactly the same position and orientation on the picture transport without irreparable loss of time. Wholly apart from this there is the serious problem in photogrammetry of processing stereo picture pairs. The individual pictures of such picture pairs are distinguished on technical photography grounds on the basis of so-called "canting," viz., they are tilted slightly toward one another. In order to be able to scan such picture pairs, it is necessary that the raster scans in both pictures be in alignment.
In addition to the geometrical problem in the case of data capture, there exists the problem of correcting variations in sensitivity of the detector elements which arise in production and even variations in the extent of illumination, so that an unequivocal relation between the detector signals and the grayscale of the scanned picture elements is obtained.
This problem is addressed in the case of West German patent 23 53 299 by a known scan apparatus in which during the scanning of a standard background or of a white card there is obtained a sensitivity profile of the detector array. This is stored in digital form and then in operation converted from its digital form into an analog form and utilized for correction of the video information signal. This method is involved and prone to error on account of the double transformation of the sensitivity profile signal.
Further methods of correction are known from U.S. Pat. No. 4,317,134 and German Patent 36 30 739, although these methods are not suited to insure a sufficiently quick and precise correction.